With the discovery of new dental filling materials, different techniques and apparatus for placing these new dental filling materials have been developed. The earliest known delivery system for use with composite resin type materials is disclosed in U.S. Pat. No. 3,581,399 issuing to Dragan on Jun. 1, 1971. Therein disclosed is a manual extruder for dispensing viscous dental material within a prepared tooth. A further improved delivery system is disclosed in U.S. Pat. No. 4,198,756 issuing to Dragan on Apr. 22, 1980. The device disclosed therein provides a mechanical advantage for the controlled dispensing of the more viscous dental materials. These devices permit improved dental filling techniques in that the viscous material is placed in the tooth cavity from the bottom up. This eliminates voids in the tooth filling and is an improvement over prior filling techniques. Since these initial disclosures of an extruding system for dispensing dental material contained in cartridges, there have been many improvements in devices to dispense the dental material as well as the containers or cartridges therefor. Most of these improvements have been directed to ease of use and efficient dispensing of the ever increasing viscous nature of newer dental materials.
Initially, a "snap-fit" front end was believed to be desirable in a dental extrusion device because of its inherent ease of use. A dental cartridge containing dental material could easily be snapped into the front end of a dental extruder. Examples of such "snap-fit" front end portions of a dental extruder are disclosed in U.S. Pat. Nos. 4,330,280 and 4,384,853. Therein disclosed is a dental extruder having flexible side walls that permit a cartridge to be snapped into the front end. The width of the front end opening is slightly less than the outside lateral width or diameter of a cartridge. Therefore, the insertion of the cartridge causes the side walls to flex, resulting in the "snap-fit". While the "snap-fit" front end dental extruders are relatively easy to use, the inherent flexibility necessary to provide a "snap-fit" results in a potentially hazardous condition. Some dental materials that are extremely viscous and placed into dental cartridges for extrusion with a dental extruder result in extremely high forces being applied to the front end. These high forces, when applied to a "snap-fit" front end often causes the flexible side walls to be forcibly spread apart during use. This results in the dental cartridge becoming wedged within the front end or even more seriously, the cartridge unintentionally shooting from the front end during use. This creates a potentially hazardous situation in that during the normal use, the front end of the dental extruder is inside a patient's mouth. Additionally, constant flexing of the side walls results in fatigue and often breakage. U.S. Pat. No. 5,061,179 entitled "Manual Extruder and Cartridge Having Interlocking Bearing Surfaces" issuing to Dragan on Oct. 29, 1991, discloses a dental extruder and cartridge attempting to overcome some of the problems encountered with the "snap-fit" front end. Therein disclosed is a dental extruder and cartridge having interlocking bearing surfaces. The interlocking bearing surfaces helped prevent the flexible side walls from spreading apart during extrusion of the more viscous dental materials. While this helped solve some of the problems inherent with a "snap-fit" front end, it did not solve the fatigue problem of repeated use and flexing of the side walls or the stress placed on the cartridge by the side walls.
Therefore, there is a need for an easy loading dental extruder with high strength and durability that can safely and confidently be used with dental materials having high viscosity.